How effortful a task feels is an integral aspect of human decision-making that shapes motivation. If a task feels very effortful we may be unwilling to perform the work required, whereas if a task feels less effortful we may be more likely to persevere. Despite the importance of these perceptions for decision-making, the behavioral and neural mechanisms of subjective effort valuation are not well understood. Furthermore, the National Institutes of Mental Health (NIMH) Research Domain Criteria (RDoC) has identified ?Effort Valuation / Willingness to Work? as a key subconstruct for understanding deficits in motivated performance in mental disorders. The goal of this proposal is to understand the mechanisms of subjective valuation of physical and cognitive effort, and the common and distinct systems that underlie these representations. To this end, we will use a combination of experiments in healthy human participants, computational modeling of behavior, and functional magnetic resonance imaging (fMRI). In Aim 1 we will identify common and distinct physical and cognitive effort valuation mechanisms. We will computationally model participants' subjective valuation of physical and cognitive effort to test if there are similarities in subjective preferences for these different types of effort. We will use model-based fMRI to examine the common and distinct brain regions that encode the subjective valuation of physical and cognitive effort, and the network of brain regions that incorporate such preferences to motivate effortful engagement. In Aim 2 we will investigate the behavioral and neural mechanisms by which physical and cognitive fatigue effect effort valuation. We will fatigue participants with sustained physical or cognitive exertion and examine how being in a fatigued state influences subjective valuation of physical and cognitive effort; and associated signals in the brain's valuation network. In Aim 3 we will explore how motivational state modulates decisions to exert physical and cognitive effort. We will pair choices for physical and cognitive effort with motivational cues (i.e., cues that formerly predicted reward) in order to modulate participants' motivational state. This manipulation will allow us to behaviorally and neurally dissociate motivation from effort valuation in order to understand how these processes interact to give rise to motivated physical and cognitive engagement. In sum, our proposed studies will have a broad impact on the field of decision-making by dissecting the behavioral and neural mechanisms responsible for physical and cognitive effort valuation. In the long term, these studies may reveal novel behavioral and neural markers to aid in the study, classification, and treatment of amotivation.